Monolith grew A LOT in the past few years, they opened a new studio and their workforce currently counts 193 employees (likely to become 200+ with these new recruitments). We know they're working on 2 games courrently in a new Action game with online components that will likely release in 2020 and the next entry in the Xenoblade series (likely not Xenoblade 3) lead by Takahashi which begun development in the second half of 2018 due to the Torna DLC. On top of that it was fairly realistic to expect them to still be involved with the upcoming 3D Zelda given the workforce they put at Nintendo's disposal for BotW, the Zelda team likely grew a lot too but they still don't have the expertese to make the OpenWorld on their own yet